Automotive coatings consist, as a rule, of an EDC (electrodeposition coating) primer layer, an optionally present primer surfacer layer (filler layer), and a top coat comprising a color- and/or special effect-imparting base coat layer and an outer protective and gloss-imparting clear coat layer. The base coat layer may consist of one single or two or more same or different base coat sublayers, i.e. the base coat sublayers may have been applied from the same base coat composition or from different, for example, differently pigmented base coat compositions.
Today, waterborne base coat compositions are state of the art and it is quite common to use aqueous polyurethane urea resin dispersions as binders therein.
A well-known method for the production of aqueous polyurethane urea resin dispersions is the so-called acetone process (see Ullmann's Encyclopaedia of Industrial Chemistry, 5th Edition, Vol. A21, pages 678-679): Typically, an NCO-functional hydrophilic polyurethane prepolymer is made by addition reaction of polyols and polyisocyanates in the presence of acetone as diluent. The NCO-functional hydrophilic polyurethane prepolymer may be reacted with a NCO-reactive chain extender like, for example, a polyamine or a hydrazine derivative. The acetone solution is converted into an aqueous dispersion by mixing with water. If a chain extension reaction is performed, it may happen before and/or after the conversion into the aqueous dispersion. Finally the acetone is removed to the desired degree, in particular, by distilling it off US 2006/0240264 A1 and US 2007/0238830 A1 disclose N-alkyl pyrrolidone-free aqueous polyurethane urea dispersions and methods for the production thereof.